Selective photocatalytic oxidation of aromatic alcohols in solar-irradiated aqueous suspensions of Pt, Au, Pd and Ag loaded TiO2 catalysts

Article abstract of DOI:10.1016/j.cattod.2016.05.054

The photocatalytic partial oxidation of 4-methoxybenzyl alcohol (MBA) and 4-nitrobenzyl alcohol (NBA) to corresponding aldehydes or acids was performed in water under simulated solar light at different pH's by using Pt, Au, Pd and Ag loaded Degussa P25 TiO₂ catalysts, prepared by photoreduction. Bare Degussa P25 TiO₂ was also used as a reference. The metal loaded TiO₂ photocatalysts were characterized by XRD, TEM, ESEM and DRS techniques. The best activity and selectivity results were obtained with Pt loaded TiO₂. The reactivity results show that metal loading on Degussa P25 sharply promotes the photoactivity and product selectivity towards aldehydes. Moreover, at low pH's very high aldehyde selectivities were determined for both alcohols. MBA oxidation rate was very high at low pH's, whereas an opposite trend was observed for NBA, due to the difference of the substituent group. Only from NBA a significant amount 4-nitrobenzoic acid (ca. 50%) was obtained at high pH values.

Full text of DOI:10.1016/j.cattod.2016.05.054

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Contents lists available at ScienceDirect
Catalysis Today
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Selective photocatalytic oxidation of aromatic alcohols in
solar-irradiated aqueous suspensions of Pt, Au, Pd and Ag loaded TiO2
catalysts
Sedat Yurdakala, Bilge Sina Teka,∗, C¸ ag˘lar Deg˘irmencib, Giovanni Palmisanoc
a Kimya Bölümü, Fen-Edebiyat Fakültesi, Afyon Kocatepe Üniversitesi, Ahmet Necdet Sezer Kampüsü, 03200, Afyonkarahisar, Turkey
b Kimya Bölümü, Fen Fakültesi, Anadolu Üniversitesi, Yunus Emre Kampüsü, Eskis¸ehir, Turkey
c Department of Chemical and Environmental Engineering, Institute Center for Water and Environment (iWater), Masdar Institute of Science and
Technology, P.O. BOX 54224, Abu Dhabi, United Arab Emirates
a r t i c l e i n f o
a b s t r a c t
Article history:
The photocatalytic partial oxidation of 4-methoxybenzyl alcohol (MBA) and 4-nitrobenzyl alcohol (NBA)
to corresponding aldehydes or acids was performed in water under simulated solar light at different pH’s
by using Pt, Au, Pd and Ag loaded Degussa P25 TiO2 catalysts, prepared by photoreduction. Bare Degussa
P25 TiO2 was also used as a reference. The metal loaded TiO2 photocatalysts were characterized by XRD,
TEM, ESEM and DRS techniques. The best activity and selectivity results were obtained with Pt loaded
TiO2. The reactivity results show that metal loading on Degussa P25 sharply promotes the photoactivity
and product selectivity towards aldehydes. Moreover, at low pH’s very high aldehyde selectivities were
determined for both alcohols. MBA oxidation rate was very high at low pH’s, whereas an opposite trend
was observed for NBA, due to the difference of the substituent group. Only from NBA a significant amount
4-nitrobenzoic acid (ca. 50%) was obtained at high pH values.
Received 22 January 2016
Received in revised form 22 May 2016
Accepted 23 May 2016
Available online xxx
Keywords:
Pt, Au, Pd and Ag loaded TiO2
pH effect
Photocatalysis
Sun light
Green synthesis
Aldehydes synthesis
© 2016 Published by Elsevier B.V.
1. Introduction
photoactivity increase occurs via the delay of the recombination
rate of photogenerated electron and hole pairs. However, a good
TiO2 is the most used photocatalyst, since it is cheap, photoac-
tive and resistant to photocorrosion [1,2]. Due to its wide band gap
energy (3.0–3.2 eV), TiO2 photocatalysts are active mainly under
UV irradiation. TiO2 may be used also under sun light because this
radiation includes ca. 3–5% UV wavelengths [3]. By considering the
energetic world strategy, and, given that sun light is a green and
sustainable energy source, photocatalysts being active under sun
light and in a green solvents as water can represent a good alter-
native to traditional thermal catalysis, usually performed at high
temperature and pressure, and by using heavy metals as catalysts
and harmful species as oxidants [2,4–7].
absorbance in visible region gives of course no warranty for better
photocatalytic activity under solar irradiation. Therefore, activity
tests for the desired oxidation need to be performed with a case-
by-case approach.
Although metal loaded TiO2 photocatalysts have been widely
used for degradation of harmful compounds under visible or
solar irradiation [13–16], only a few investigations were carried
out on their application in organic syntheses [17–19]. Shi-
raishi and co-workers investigated the photocatalytic synthesis of
nitrosobenzene from aniline (90%) by using Pt loaded TiO2 photo-
catalysts in toluene and under visible irradiation [19].
In order to increase TiO2 photoactivity under sun light, load-
ing by metals has been pursued [8–12]. This procedure affords
absorbance in the visible region, so that both UV and visible radi-
ation can activate the semiconductor catalyst [12]. In other words,
metal loading can introduce intraband gap states, thus improv-
ing its ability to absorb low energy photons. In addition, the
In that work, home prepared anatase and rutile TiO2 photocat-
alysts were prepared loaded by Pt. Degussa P25 was also used for
comparison. All Pt-loaded catalysts have significant absorbance in
visible irradiation. Degussa P25 showed negligible photoactivity
under visible irradiation due to the lack of absorbance of visible
radiation. Zheng et al. [8] reported on the preparation of Au, Pt and
Ag loaded home made TiO2 by using a novel method and applied
them to the selective photocatalytic oxidation of benzene to phe-
nol in aqueous suspension and under visible irradiation. The metal
loading procedure involved Ti4+ in TiO2 being reduced to Ti3+in
∗
Corresponding author.
E-mail address: sinabilge.t@gmail.com (B.S. Tek).
http://dx.doi.org/10.1016/j.cattod.2016.05.054
0920-5861/© 2016 Published by Elsevier B.V.
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suspensions of Pt, Au, Pd and Ag loaded TiO2 catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.054

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Fig. 1. The irradiation spectrum of solar simulator (1500W) (a) and used reactors (b) [4].
ethanol and, finally, the addition of a noble metal salt with the
reduction of the noble metal cation carried out under dark by Ti3+
solution, AgNO3, AuCl or PdNO3) was prepared. This suspension
was then treated by ultrasounds for 15 min. Given the low solubility
of AuCl, a stoichiometric amount of NaCl was added to the solution
.
High yield (63%) and selectivity (91%) for phenol production were
obtained. A mechanism was suggested for oxidation of benzene on
Au-loaded TiO2 in which an electron transfers from Au nanopar-
ticles to TiO2 under visible irradiation, and Au+ oxidizes phenoxy
anions to form phenoxy radicals finally producing phenol.
to promote the formation of the soluble complex [AuCl2] −
.
For the photodeposition of the metals on TiO2 an annular 2.5 L
Pyrex batch photoreactor was used; it was filled with the above
described suspension. The gas-seal photoreactor was provided with
ports in its upper section for the inlet and outlet of gases. A magnetic
bar guaranteed a satisfactory suspension of the photocatalyst and
the uniformity of the reacting mixture. A 700 W medium pressure
Hg lamp (Helios Italquartz) was axially immersed within the pho-
toreactor and it was cooled by water circulating through a Pyrex
thimble, so as to keep the temperature of the suspension at ca.
300 K. The aqueous suspension was continuously bubbled with
Helium at atmospheric pressure; the gas was introduced in the
reactor 30 min before turning the lamp on. The gas, the lamp and
the agitation were maintained for 8 h. After waiting for the solid
decantation, the liquid phase was separated from solid phase and
the so obtained wet catalyst was washed two times by using 2.5 L
batches of deionised water. Finally, the wet powder was dried at
100 ◦C and a fine powder was obtained. After drying, some batches
were calcined in air at 400 ◦C for 3 h. Hereafter the catalysts are
labelled as Me-TiO2-X%-Y, being Me the loaded metal, X its molar
percentage and Y the treatment temperature (◦C).
XRD patterns of the powders were recorded by a Philips
diffractometer using the Cu K␣ radiation and a 2␪ scan rate of
1.281◦ min−1. SEM images were obtained using an ESEM micro-
scope (Philips, XL30) operating at 25 kV. A thin layer of gold was
evaporated on the catalysts samples, previously sprayed on the
stab from a suspension and dried at room temperature. UV–vis
spectra were obtained by diffuse reflectance spectroscopy using
a Shimadzu UV-2401 PC instrument. BaSO4 was used as a refer-
ence and the spectra were recorded in the 300–600 nm range. TEM
images were obtained using an HR-TEM instrument (JEM-2100).
Before the analysis the samples were dispersed in deionised water
by ultrasound treatment, deposited on a carbon-coated Cu grid, and
dried at room temperature.
Due to both low solubility of most of organic molecules in water
and, also, to the generally low selectivity of the photocatalytic
organic synthetic processes in water, photocatalytic synthetic reac-
tions have been generally performed in organic solvents [20].
Selective oxidation of alcohol to carbonyl groups can be con-
sidered a key step in many organic syntheses [20]. The common
practice to perform this oxidation usually consists in a catalytic
process carried out in the presence of organic solvents at high
temperature and pressure making use of stoichiometric oxygen
donors that not only are expensive and toxic, but also produce large
amounts of dangerous waste [2,4–7]. Moreover, huge amounts
of heavy metals are generally used as catalysts. The elimination
of these environmentally harmful conditions is, thus, one of the
challenges of nowadays research. Recently, photocatalytic partial
oxidation of aromatic alcohols to the corresponding aldehydes has
been carried out under solar radiation in aqueous suspensions of
home-prepared (HP) anatase and rutile TiO2. In those cases HP pho-
tocatalysts, containing high amounts of amorphous phase, showed
selectivity values for aldehyde production far higher than those of
commercial and crystalline samples [20–23].
In the present work, the photocatalytic oxidation of 4-
methoxybenzyl alcohol (MBA) and 4-nitrobenzyl alcohol (NBA)
was performed in water and under simulated solar light at different
pH’s. In order to increase TiO2 activity and product selectivity, Me-
loaded TiO2 catalysts (Me = Pt, Au, Pd and Ag) were home-prepared
by photoreduction. Degussa P25 [24,25] and metal loaded Degussa
P25 [19,26–28] were deeply investigated by many studies. The spe-
cific aim of the present work is to evaluate the influence of the
different metals loaded on TiO2, of the suspension pH and of the
substituent groups of benzyl alcohol on the performance of the pho-
tocatalytic process. The catalysts were characterised by XRD, TEM,
ESEM and DRS techniques and their photoactivity was assessed by
runs carried out in a batch reactor under sun light irradiation.
2.2. Photoreactivity setup and procedure
The photoreactor used for the runs carried out under simu-
lated solar radiation was a 250 mL cylindrical beaker (diameter:
6.7 cm) containing 150 mL of aqueous suspension. A magnetic stir-
rer guaranteed a satisfactory suspension of the photocatalyst and
the homogeneity of the reacting mixture. This photoreactor was
placed inside a 1500 W solar light simulator equipped with a Xenon
lamp and a reflector. The distance from the lamp to the suspension
2. Experimental
2.1. Catalyst preparation and characterization
A suspension made of 2 L water, 500 mL ethanol, 10 g of Degussa
P25 and the required amount of Pt, Ag, Au or Pd sources (PtCl4
Please cite this article in press as: S. Yurdakal, et al., Selective photocatalytic oxidation of aromatic alcohols in solar-irradiated aqueous
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top level was 27.5 cm. The values of the radiation energy imping-
ing on the suspension, measured by using a radiometer (Delta
Ohm, DO 9721), were 0.81 mW cm−2 in the 315–400 nm range and
95 mW cm−2 in the 400–1050 nm range. All the experiments were
carried out at 30 ◦C. Fig. 1 shows the irradiation spectrum of the
solar simulator (1500W) and the used photoreactors.
Ag-TiO
2
-0.5%-100
For all the runs the alcohol initial concentration was 0.60 mM;
the pH was adjusted before each run at 1, 7 and 13 by employ-
ing HCl and NaOH solutions. The catalyst amount was 0.20 g L−1
;
this amount guaranteed that all the catalyst particles were irra-
diated. Before switching the lamp on, the suspension was stirred
for 30 min at room temperature in order to reach the thermody-
namic adsorption equilibrium. The contact of the suspension with
the atmospheric air guaranteed that the aqueous solution was sat-
urated by oxygen. During the run, samples of reacting suspension
were withdrawn at fixed time intervals; they were immediately
filtered through a 0.45 ␮m hydrophilic membrane (HA, Millipore)
before being analysed.
Pt-TiO
2
-0.5%-100
Au-TiO
2
-0.5%-100
The quantitative determination and identification of the species
present in the reacting suspension were performed by means of a
Shimadzu HPLC (Prominence LC-20A model and SPD-M20A Pho-
todiode Array Detector), equipped with a Phenomenex Synergi
4 ␮m Hydro-RP 80A column at 313 K, using Sigma Aldrich stan-
dards. Retention times and UV–vis spectra of the compounds were
compared with those of standards. The eluent consisted of: 50%
methanol and 50% 13 mM trifluoroacetic acid aqueous solution.
The flow rate was 0.40 cm3 min−1. All the used chemicals were
purchased from Sigma Aldrich with a purity of at least 98.0%.
Pd-TiO
2
-0.5%-100
10
20
30
40
50
60
70
80
2ɵ/°
3. Results and discussion
Fig. 2. XRD patterns of prepared metal loaded Degussa P25 photocatalysts.
3.1. Catalyst characterization
lyst and oxygen were all needed for the oxidation of the substrates
and the catalysts were stable in cycles of repeated runs, as expected
for stable samples such as TiO2 P25 and noble metals in their zero-
oxidation state. In homogenous conditions, i.e. in the absence of
catalyst, a negligible activity was observed.
Adsorption of the alcohols under dark conditions was always
quite low, i.e. less than 3%. For all the used catalysts the main prod-
ucts of alcohol photocatalytic oxidation were the corresponding
aldehydes (p-anisaldehyde, (PAA) or p-nitrobenzaldehyde), acids
(p- methoxybenzoic acid or p-nitrobenzoic acid) and CO2.
The experimental results obtained at different pH’s for the
MBA oxidation by metal loaded and un-loaded TiO2 catalysts are
reported in Table 1 as the half-life time of MBA, the selectivity
to aldehyde calculated for 50% conversion. Selectivity values were
calculated as the ratio between the produced moles of products
(aldehyde or acid) and the reacted moles of substrate.
Unexpectedly, very selective (ca.100%)and fast PAA synthesis
from MBA have been achieved at pH = 1. The reaction rates of
the reaction performed in neutral (pH = 7) and basic conditions
(pH = 13) are smaller and close to each other, and the selectivities
are lower than that at pH = 1 (Table 1).
Activity and selectivity results of Ag- and Pt-loaded TiO2 cata-
lysts in neutral and basic conditions are close to un-loaded TiO2,
while at pH = 1 metal loaded ones are more active than un-loaded
Degussa P25. In acidic medium Au- and Pd-loaded TiO2 showed
activity and selectivity very similar to bare TiO2, while under neu-
tral and basic conditions they showed worse activity and selectivity
than those of Degussa P25. These results indicate that Au and Pd
loadings do not improve TiO2 catalytic activity for PAA synthesis.
Pt-TiO2-0.5%-100 showed the best performance, by assessing
both selectivity and activity. In order to investigate the influence
of the treatment temperature on the catalyst activity, this catalyst
was also calcined at 400 ◦C for 3 h. The Pt-TiO2-0.5%-400 sample
showed better results than Pt-TiO2-0.5%-100, most probably due
XRD patterns of pristine and metal loaded Degussa TiO2 pho-
tocatalysts showed the same crystalline phases, that were not
influenced by the photodeposition of the metals, as expected
(Fig. 2). The XRD patterns show that Degussa P25 is a well-
crystallized mixture of anatase and rutile phases as indicated by
the sharp diffraction peaks [4]. The peaks at 2␪ = 25.58◦, 38.08◦,
48.08◦ and 54.58◦ are characteristic of anatase phase and those at
2␪ = 27.51◦, 36.51◦, 41.1◦, 54.11◦ and 56.51◦ of rutile [4]. The diffrac-
tion peaks of the metals are not detectable because of their limited
amount (≤1%).
The primary crystallite sizes of all the samples are 26.5 nm
for anatase and 18.2 nm for rutile crystal (calculated by using
the Scherrer equation). These figures are in agreement with TEM
observations (Fig. 3), although a limited occurrence of bigger
nanoparticles (up to diameters of ca. 70 nm) should also be noticed.
The TEM images (Fig. 3) show that the metals loaded on TiO2 cata-
lysts are well distributed on the catalyst surface; the size of metal
particles is in the 1÷3 nm range. The size of the aggregates is in
the 40–70 nm range, as determined by SEM observations (Fig. 4).
Both TEM and SEM observations did not highlight noteworthy dif-
ferences among the different samples.
UV–vis spectra of the metal loaded photocatalysts are reported
in Fig. 5; Degussa P25 spectrum is also reported for comparison.
The spectra show that all the samples have a good absorbance
under UV irradiation, while just metal loaded samples have also
good absorbance under visible irradiation.
3.2. Photoreactivity
No oxidation of alcohols was observed in the absence of irradi-
ation or oxygen for runs carried out under the same experimental
conditions employed for the photocatalytic ones. Irradiation, cata-
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Fig. 3. TEM images of Pt- TiO2-0.5%-100 (a), Pt-TiO2-0.5%-400 (b) Au-TiO2-0.5%-100 (c), Pd- TiO2-0.5%-100 (d), Ag-TiO2-0.5%-100 (e).
to an improved contact between the loaded metal particles and
TiO2. By using this catalyst, 71% selectivity for PAA production was
obtained even at neutral pH with a higher activity (t1/2 = 1.6 vs.
2.1 h). For all the used catalysts and in any experimental condi-
tions, the obtained 4-methoxybenzoic acid selectivity was always
very low (<1%).
The Pt-TiO2-0.5%-400 catalyst has also been used for the NBA
oxidation runs, which were carried out in order to investigate the
influence of the substituent group of the aromatic ring on the
photoactivity. Fig. 6 shows experimental results of photocatalytic
oxidation of NBA to NBAD and 4-nitrobenzoic acid by using Pt-TiO2-
0.5%-400 photocatalyst at pH = 13. This Figure reports the NBA,
NBAD and 4- nitrobenzoic acid concentrations versus the irradi-
Please cite this article in press as: S. Yurdakal, et al., Selective photocatalytic oxidation of aromatic alcohols in solar-irradiated aqueous
suspensions of Pt, Au, Pd and Ag loaded TiO2 catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.054

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Fig. 4. SEM image (magnification: x200000) of Ag-TiO2-%0.5-100 (a), Pt-TiO2-%0.125-100 (b), Pt-TiO2- %0.25-100 (c), Pt-TiO2-%0.5-100 (d), Pt-TiO2-%0.5-400 (e) photocata-
lysts.
ation time; the calculated values of NBA conversion and selectivity
to NBAD and 4-nitrobenzoic acid are illustrated as well. It may be
noted that, by increasing the alcohol conversion, the aldehyde pro-
duction also increases up to a 30% conversion, afterwards it starts
decreasing due to overoxidation of NBAD to 4-nitrobenzoic acid.
For this reason, aldehyde selectivity continuously decreases, while
acid selectivity increases. The experimental results obtained at dif-
ferent pH’s for the NBA oxidation by Pt-TiO2-0.5%-400 and Degussa
P25 catalysts are reported in Table 2, along with the half- life time of
NBA, the selectivity to aldehyde and to acid calculated for 50% con-
version. To the sake of comparison, Table 2 also reports the results
obtained with MBA. From these data one can conclude that Pt-TiO2-
0.5%-400 produced better results than bare TiO2, for both MBA and
NBA oxidations. In particular, higher selectivities were recorded in
the formation of the corresponding aldehydes for both alcohols,
especially in acidic media. For instance, at pH = 1, PAA selectivity
was 99%, while 4-nitrobenzaldehyde (NBAD) selectivities were 75%
and 85% for bare TiO2 and Pt- TiO2-0.5%-400, respectively. How-
Please cite this article in press as: S. Yurdakal, et al., Selective photocatalytic oxidation of aromatic alcohols in solar-irradiated aqueous
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Table 1
0.6
60
50
40
30
20
10
0
Photoreactivity results obtained under solar irradiation. MBA initial concentration:
0.6 mM; catalyst amount: 0.20 g L−1. t1/2 and selectivity values were calculated for
50% conversion.
0.5
0.4
0.3
0.2
0.1
0
Catalyst
pH
t1/2(h)
Aldehyde Selectivity (%)
Homogenous
Degussa P25
Degussa P25
Degussa P25
1
1
7
13
negligible conv.
0.83
2.0
100
15
27
1.7
Au-TiO2-0.5%-100
Au-TiO2-0.5%-100
Au-TiO2-0.5%-100
1
7
13
0.80
3.0
2.3
100
17
32
0
0.5
1
1.5
2
2.5
3
t (h)
Pd-TiO2-0.5%-100
Pd-TiO2-0.5%-100
Pd-TiO2-0.5%-100
1
7
13
0.70
3.9
2.6
100
30
29
Fig. 6. Experimental results of photocatalytic oxidation of NBA ( ) to NBAD (
and 4-nitrobenzoic acid ( ) by using Pt- TiO2-0.5%-400 (0.2 g/L) photocatalyst at
pH = 13. Conversion ( ), selectivity to NBAD ( ) and selectivity to 4-nitrobenzoic
acid ( ) values are quoted on the right ordinate axis.
)
Ag-TiO2-0.5%-100
Ag-TiO2-0.5%-100
Ag-TiO2-0.5%-100
Ag-TiO2-1%-100
Ag-TiO2-1%-100
Ag-TiO2-1%-100
1
7
13
1
7
0.53
3.5
1.8
0.75
2.5
2.1
100
15
12
100
13
21.5
13
ever, at pH = 1, the reaction rate of NBA oxidation was very low.
After 3 h irradiation, just 19% and 14% conversions were obtained
by using the bare and loaded catalyst, respectively.
Pt-TiO2- 0.5%-100
Pt-TiO2- 0.5%-100
Pt-TiO2- 0.5%-100
Pt-TiO2- 0.25%-100
Pt-TiO2- 0.125%-100
1
7
13
1
1
0.5
2.1
2.5
0.45
0.45
100
30
33
100
85
The reactivity for NBA oxidation grows by increasing pH. In
addition, along with NBAD a significant amount of 4-nitrobenzoic
acid was also obtained. NBAD selectivity was very high (52–53%)
by using both catalysts at pH = 13. From the results reported in
Table 2, it may be noted that at pH = 1 the total selectivity (for
both aldehyde and acid production) is 100% for Pt-TiO2-0.5%-400
and 88% for Degussa P25. Similarly, at pH = 13 a high total selec-
tivity was obtained. However, at neutral conditions, Degussa P25
showed markedly a low total selectivity (19%), while Pt-TiO2-0.5%-
400 showed a total selectivity value of 69%. These results show that
Pt loading improves not only the reaction rate, but also the prod-
uct selectivity in these green conditions, which importantly do not
require modifying the pH to values far from the neutrality.
Pt-TiO2-0.5%-400
Pt-TiO2-0.5%-400
Pt-TiO2-0.5%-400
Pt-TiO2-1%-400
1
7
13
1
0.45
1.6
2.1
1
100
71
30
95
1.6
1.4
1.2
1
Pd-TiO
Pt-TiO
Ag-TiO
Au-TiO2- 0.5%100
Degussa P25
2
-0.5%-100
-0.5%100
-0.5%100
2
2
The different reactivity results obtained with MBA and NBA can
be analyzed by considering the different nature of the substituent
groups in MBA and NBA. MBA includes an electron donor group
(methoxy), whereas NBA carries a nitro-group, which is electron
withdrawing, and this difference could be a reason of the different
performance highlighted in Table 2. In the presence of an electron
donor group, as methoxy, the reaction rate is fast in acidic medium,
whereas in the presence of electron withdrawing group, as nitro,
the reaction rate is fast in basic medium. Moreover, in the presence
of a nitro group, unlike the case of methoxy, a significant amount of
4-nitrobenzoic acid was obtained. The amount of this acid is very
high especially in highly basic conditions, owing to the fact that OH
radicals, prevalently formed in a basic medium, crucially promote
0.8
0.6
0.4
0.2
0
300
350
400
450
500
550
600
ʎ (nm)
Fig. 5. UV–vis diffuse reflectance spectra of Pd, Pt, Ag, and Au loaded TiO2 photo-
catalysts.
Table 2
Photoreactivity results obtained under solar irradiation. Aromatic alcohol initial concentration: 0.60 mM; catalyst amount: 0.20 g L−1. t1/2 and selectivity values were calculated
for 50% conversion.
Catalyst
Substrate
pH
t1/2 (h)
AldehydeSelectivity (%)
AcidSelectivity (%)
Total product selectivity (%)
Degussa P25
Degussa P25
Degussa P25
MBA
MBA
MBA
1
7
13
0.83
2.0
1.7
100
50
27
–
0.6
0.7
100
15
28
Degussa P25a
Degussa P25
Degussa P25
NBA
NBA
NBA
1
7
13
%19 Conv.
5.0
2.1
75
18
30
13
1.2
53
88
19
83
Pt-TiO2-0.5%-400
Pt-TiO2-0.5%-400
Pt-TiO2-0.5%-400
MBA
MBA
MBA
1
7
13
0.45
1.6
2.1
100
71
30
–
0.8
0.5
100
72
31
Pt-TiO2−0.5%-400a
Pt-TiO2-0.5%-400
Pt-TiO2-0.5%-400
NBA
NBA
NBA
1
7
13
%14 Conv.
5.3
3.4
85
55
21
14
14
52
99
69
73
a
These values were calculated for 3 h of irradiation due to their low conversion.
Please cite this article in press as: S. Yurdakal, et al., Selective photocatalytic oxidation of aromatic alcohols in solar-irradiated aqueous
suspensions of Pt, Au, Pd and Ag loaded TiO2 catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.054

G Model
CATTOD-10285; No. of Pages7
ARTICLE IN PRESS
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7
its formation. On the contrary, NBAD formation is more preferred
in acidic medium. Unlike NBA oxidation, a negligible amount of 4-
methoxybenzoic acid was obtained from MBA oxidation. This result
can be explained by considering that the main oxidation prod-
uct of MBA, i.e. PAA, mainly degrades to open ring products and
CO2, rather than undergoing oxidation to 4-methoxybenzoic acid,
as demonstrated by ad hoc tests performed starting with PAA as the
reactant. Finally, we can notice that nitro-group is also responsible
for the decrease of the reactivity, in agreement with recent pub-
lished evidence [21,22]. Previous research works reported that the
mineralization of adsorbed aromatic alcohol molecules proceeds
through steps in which the oxidation products are not released into
the liquid phase, suggesting that this process should be favoured
by using a catalyst with a hydrophobic surface [20,23]. However, in
the present work, the experimental results show that by adjusting
pH, green synthesis is achievable selectively even on TiO2 catalysts
owning a given hydrophobic character.
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Pt, Au, Ag and Pd loaded Degussa P25 TiO2 samples were pre-
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The article is dedicated to the career of prof. Vincenzo
Augugliaro (University of Palermo, Italy) in the occasion of his
retirement.
Authors thank the TÜBITAK (Project no: 111T489) for financial
support.
˙
Authors thank for TEM experimental data were provided by Cen-
tro Grandi Apparecchiature—UniNetLab—Università di Palermo.
Please cite this article in press as: S. Yurdakal, et al., Selective photocatalytic oxidation of aromatic alcohols in solar-irradiated aqueous
suspensions of Pt, Au, Pd and Ag loaded TiO2 catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.054